1. Field of the Invention
This invention relates to conductive polymer compositions, methods of making such compositions, and electrical devices comprising such compositions.
2. Introduction to the Invention
Conductive polymer compositions and electrical devices comprising them are well known. Such compositions comprise a polymer and, dispersed in the polymer, a particulate conductive filler. The type and quantity of the conductive particles, as well as the type of the polymer, influence the resistivity of the composition. For compositions with resistivities greater than about 1 ohm-cm, carbon black is a preferred filler. For compositions with lower resistivities, metal particles are used. Compositions comprising carbon black are described, for example, in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al.), 4,388,607 (Toy et al.), 4,534,889 (van Konynenburg et al.), 4,560,498 (Horsma et al.), 4,591,700 (Sopory), 4,724,417 (Au et al.), 4,774,024 (Deep et al.), 4,935,156 (van Konynenburg et al.), and 5,049,850 (Evans et al.). Compositions comprising metal fillers are described, for example, in U.S. Pat. Nos. 4,545,926 (Fouts et al.), 5,250,228 (Baigrie et al.), and 5,378,407 (Chandler et al.). The disclosure of each of these patents is incorporated herein by reference.
The electrical properties of conductive polymer composition tend to deteriorate over time. For example, in metal-filled conductive polymer compositions, the surfaces of the metal particles tend to oxidize when the composition is in contact with an ambient atmosphere, and the resultant oxidation layer reduces the conductivity of the particles when in contact with each other. The electrical performance of devices containing conductive polymer compositions can be improved by minimizing the exposure of the composition to oxygen. One approach is to cover some or all of the composition with a protective layer.
Oxygen barrier layers for positive temperature coefficient (PTC) devices are described, for example, in U.S. Pat. No. 4,315,237 (Middleman et al.). One type of barrier layer contains a physical barrier material, such as a conventional epoxy composition, silicone resin, or insulating tape. Another type of barrier layer contains an oxygen barrier material, which exhibits oxygen permeabilities that are at least an order of magnitude lower than those of physical barrier materials. Examples of oxygen barrier materials include polyvinyl alcohol (PVOH), poly(ethylene-co-vinyl alcohol) (EVOH), poly(ethylene naphthalate) (PEN), poly(vinylidene chloride) (PVDC) and polyacrylonitrile (PAN).
One disadvantage common to the oxygen barrier materials is that they are typically processed by thermoplastic processing techniques, in which the polymer is melted and applied to another substance while in the molten form. The high temperatures and special equipment necessary for processing these polymers can hinder their use as coatings for PTC devices, since it is difficult to form an acceptable seal between the oxygen barrier and the electrodes in the device. Solutions or emulsions of these polymers tend to have prohibitively high viscosities and/or to contain solvents or other additives that could damage one or more components of the device. Both of these techniques, when used with these conventional barrier polymers, can present difficulties in consistently forming adequate sealing between the barrier and the electrodes. Another disadvantage to some of these oxygen barrier polymers is their tendency to absorb water from the environment. This is particularly problematic for PVOH and EVOH, with the result that the oxygen permeability of these polymers is much greater when in an environment of high relative humidity.
It is desirable to provide electrical devices that can be protected from oxidation on a consistent basis and under a variety of environmental conditions. It is also desirable that any material used to protect the device could be applied through straightforward processing techniques.